This invention relates to casting alloys particularly useful in directional solidification and, more particularly, to such an alloy structure useful in the tip portion of gas turbine engine blades.
During operation of axial flow turbine engines, for example gas turbine engines, very close tolerances are maintained between the tips of blading members and opposed cooperating members assembled in a type of gas seal. Such a seal is intended to inhibit leakage of gas, for example compressed air or combustion products, about the blade tips. Because of the difference in rates of thermal expansion of such cooperating members, interference between rotating and stationary parts can occur. This problem is more difficult in the turbine portion of the engine because of the higher temperatures experienced. In addition, because of such elevated temperatures, oxidation resulting from the presence of air and sulfidation resulting from airborne corrosive compounds such as sea salt further complicate the problem. As a result, a variety of coatings for gas turbine blades have been developed and reported. However, during interference between a rotating and a stationary component in the turbine of such an engine, the coating is rubbed away at the blade tip exposing the alloy to oxidation and sulfidation. In general, nickel-base superalloys possess good oxidation properties and relatively poor corrosion or sulfidation resistance. Conversely, cobalt-base superalloys used for such turbine blades generally possess good sulfidation or corrosive properties but poorer oxidation resistance. Accordingly, there is a need for a turbine blade tip alloy capable of long life in both oxidation and corrosive atmospheres.